Add ./src/types.ts to generated TypeScript CLI

[jamonholmgren]

Per #451, for TypeScript generated CLIs, I'd like to add a ./src/types.ts file that you export your public-facing types from, plus these:

tsconfig.json:

  "compilerOptions": {
    "declaration": true,
    "declarationDir": "build/types",

package.json:

  "types": "build/types/types.d.ts",

[JessWallin]

Hey! Put up a PR for this. Let me know if this is what you're looking for, I'm happy to revise.